Data from the FDR indicated that the aircraft did not exceed any limitations prescribed in the AFM, yet propeller operating disturbances occurred. Although the static engine pitch angle limitation was unknowingly exceeded and might have contributed to a simultaneous temporary loss of engine oil pressure in both engines, the resolution of acceleration vectors showed that the effect of the pitch angle was equivalent to a steady-state pitch-up angle within limitations during the manoeuvre. Low oil pressure conditions are known to occur during negative g manoeuvres with the original PRV but the low oil pressure condition did not last long enough to affect propeller operation. Data suggest that the modified PRV contributed to the propeller speed disturbance by prolonging the low oil pressure conditions. This result has been replicated in subsequent flight testing involving push-overs to about -0.10g from positive pitch angles of about 23and demonstrates that propeller operating disturbances can be anticipated any time that negative g conditions are encountered. This previously unknown consequence is the subject of discussions between the manufacturer andTC. Section 2.2.5 of the AFM, Manoeuvring Limit Load Factors, indicates that the allowable load factors limit the permissible bank angle in turns and limit the severity of pull-up and push-over manoeuvres. Bank angles were not an issue in this incident, and it is true that load factor limits would restrict the severity of a pull-up or push-over manoeuvre. While the flight test load factor targets were within the limits specified in the AFM, the AFM is not produced as a flight test design guide. Certification flight testing programs could involve manoeuvres considered to be extreme for the category of aircraft. The Cascade Aerospace Inc. flight test engineering group did not know exactly what pitch angle would be achieved during the planned series of manoeuvres. Therefore, it would be prudent for engineering groups to fully research the limits of systems, such as lubrication and fuel systems, when designing such flight testing programs and incorporate such information in the flight test cards applicable to each flight. The airframe manufacturer concluded that the pitch-up attitude achieved was extreme and is not likely to be encountered in normal operations for this type of aircraft. However, a query of another DHC-8 operator (not a Q400model) indicated that high positive pitch angles (30or more) could be achieved in flight training manoeuvres. Therefore, it is not beyond reason for load factors of less than 0g to be induced during a training exercise or actual traffic or terrain avoidance manoeuvre. Without knowledge of the associated consequences, an already abnormal manoeuvre could involve much higher risks if an unsuspecting flight crew encountered a single or double loss of engine oil pressure and subsequent propeller overspeed condition(s). The original certification flight test demonstrations conducted by Bombardier did not result in any abnormalities of propeller operation with the original PRV installation in the engines. Since the introduction of the modified PRV (SB35038, Revision3), this in-flight incident and subsequent test cell results demonstrate that the loss of engine oil pressure was prolonged by the modified PRV and resulted in the propeller underspeed when the blades were driven in the coarse pitch direction. This effect of the PRVmodification had not been foreseen. When this event occurred, the balance of the system performed as designed as evidenced by the recovery of the No.1 engine/propeller to its prior operating state. Because the engines were operating at less than 50percent torque, had it not been for the increase in torque due to the advancement of the No.2 power lever- which completed the conditions required for activation of the AUPC- the No.2 engine/propeller system likely would have recovered to its prior operating state as well. Had the engines been operating at more than 50percent torque, both propellers could have been forced to operate on the overspeed governors. This is not a critical event, and the aircraft could have continued flight to an aerodrome under this condition. Following an intentional in-flight shutdown and subsequent in-flight re-start of the No.2 engine, the crew found that the propeller could not be unfeathered. None of the aircraft manuals or training material, nor the FlightSafety International pilot training course inform flight crews that, when the PEC detects a failure condition that requires propeller feathering, the ability to subsequently unfeather the propeller will be lost, even in the case of a greater emergency. Just such a greater emergency occurred with this same aircraft about 30flight hours later (TSB report A05P0137) when the No.1 engine suddenly shut down in flight without warning.Analysis Data from the FDR indicated that the aircraft did not exceed any limitations prescribed in the AFM, yet propeller operating disturbances occurred. Although the static engine pitch angle limitation was unknowingly exceeded and might have contributed to a simultaneous temporary loss of engine oil pressure in both engines, the resolution of acceleration vectors showed that the effect of the pitch angle was equivalent to a steady-state pitch-up angle within limitations during the manoeuvre. Low oil pressure conditions are known to occur during negative g manoeuvres with the original PRV but the low oil pressure condition did not last long enough to affect propeller operation. Data suggest that the modified PRV contributed to the propeller speed disturbance by prolonging the low oil pressure conditions. This result has been replicated in subsequent flight testing involving push-overs to about -0.10g from positive pitch angles of about 23and demonstrates that propeller operating disturbances can be anticipated any time that negative g conditions are encountered. This previously unknown consequence is the subject of discussions between the manufacturer andTC. Section 2.2.5 of the AFM, Manoeuvring Limit Load Factors, indicates that the allowable load factors limit the permissible bank angle in turns and limit the severity of pull-up and push-over manoeuvres. Bank angles were not an issue in this incident, and it is true that load factor limits would restrict the severity of a pull-up or push-over manoeuvre. While the flight test load factor targets were within the limits specified in the AFM, the AFM is not produced as a flight test design guide. Certification flight testing programs could involve manoeuvres considered to be extreme for the category of aircraft. The Cascade Aerospace Inc. flight test engineering group did not know exactly what pitch angle would be achieved during the planned series of manoeuvres. Therefore, it would be prudent for engineering groups to fully research the limits of systems, such as lubrication and fuel systems, when designing such flight testing programs and incorporate such information in the flight test cards applicable to each flight. The airframe manufacturer concluded that the pitch-up attitude achieved was extreme and is not likely to be encountered in normal operations for this type of aircraft. However, a query of another DHC-8 operator (not a Q400model) indicated that high positive pitch angles (30or more) could be achieved in flight training manoeuvres. Therefore, it is not beyond reason for load factors of less than 0g to be induced during a training exercise or actual traffic or terrain avoidance manoeuvre. Without knowledge of the associated consequences, an already abnormal manoeuvre could involve much higher risks if an unsuspecting flight crew encountered a single or double loss of engine oil pressure and subsequent propeller overspeed condition(s). The original certification flight test demonstrations conducted by Bombardier did not result in any abnormalities of propeller operation with the original PRV installation in the engines. Since the introduction of the modified PRV (SB35038, Revision3), this in-flight incident and subsequent test cell results demonstrate that the loss of engine oil pressure was prolonged by the modified PRV and resulted in the propeller underspeed when the blades were driven in the coarse pitch direction. This effect of the PRVmodification had not been foreseen. When this event occurred, the balance of the system performed as designed as evidenced by the recovery of the No.1 engine/propeller to its prior operating state. Because the engines were operating at less than 50percent torque, had it not been for the increase in torque due to the advancement of the No.2 power lever- which completed the conditions required for activation of the AUPC- the No.2 engine/propeller system likely would have recovered to its prior operating state as well. Had the engines been operating at more than 50percent torque, both propellers could have been forced to operate on the overspeed governors. This is not a critical event, and the aircraft could have continued flight to an aerodrome under this condition. Following an intentional in-flight shutdown and subsequent in-flight re-start of the No.2 engine, the crew found that the propeller could not be unfeathered. None of the aircraft manuals or training material, nor the FlightSafety International pilot training course inform flight crews that, when the PEC detects a failure condition that requires propeller feathering, the ability to subsequently unfeather the propeller will be lost, even in the case of a greater emergency. Just such a greater emergency occurred with this same aircraft about 30flight hours later (TSB report A05P0137) when the No.1 engine suddenly shut down in flight without warning. During the flight test manoeuvre, both aircraft engines developed a loss of oil pressure when exposed to a high pitch-up angle along with a brief and small negative g condition. When exposed to low oil pressure conditions, the modified engine oil pressure regulating valve allowed the low oil pressure condition to exist long enough to affect propeller operation. The extended low oil pressure condition resulted in propeller speed fluctuations, and caused both propellers to enter an underspeed condition. This fulfilled one of three requirements for the subsequent No.2 propeller overspeed condition. While the No. 2 propeller was in the underspeed condition, advancement of the No.2 power lever resulted in the engine torque exceeding 50percent. This fulfilled the second requirement for the subsequent No.2 propeller overspeed condition. The occurrence of these two conditions simultaneously for more than one second completed all of the requirements for activation of the No.2 automatic underspeed protection circuit (AUPC) and resulted in the No.2 propeller overspeed condition.Findings as to Causes and Contributing Factors During the flight test manoeuvre, both aircraft engines developed a loss of oil pressure when exposed to a high pitch-up angle along with a brief and small negative g condition. When exposed to low oil pressure conditions, the modified engine oil pressure regulating valve allowed the low oil pressure condition to exist long enough to affect propeller operation. The extended low oil pressure condition resulted in propeller speed fluctuations, and caused both propellers to enter an underspeed condition. This fulfilled one of three requirements for the subsequent No.2 propeller overspeed condition. While the No. 2 propeller was in the underspeed condition, advancement of the No.2 power lever resulted in the engine torque exceeding 50percent. This fulfilled the second requirement for the subsequent No.2 propeller overspeed condition. The occurrence of these two conditions simultaneously for more than one second completed all of the requirements for activation of the No.2 automatic underspeed protection circuit (AUPC) and resulted in the No.2 propeller overspeed condition. Any load factors below 0g, such as during a training exercise or an actual traffic or terrain avoidance manoeuvre, could involve much higher risks if an unsuspecting flight crew became distracted by a second unexpected event such as a single or double loss of engine oil pressure and subsequent propeller overspeed condition(s). None of the aircraft manuals or training material, nor the FlightSafety International pilot training course, inform flight crews that, when the propeller electronic control (PEC) detects a failure condition that requires propeller feathering, the ability to subsequently unfeather the propeller will be lost, even in the case of a greater emergency.Findings as to Risk Any load factors below 0g, such as during a training exercise or an actual traffic or terrain avoidance manoeuvre, could involve much higher risks if an unsuspecting flight crew became distracted by a second unexpected event such as a single or double loss of engine oil pressure and subsequent propeller overspeed condition(s). None of the aircraft manuals or training material, nor the FlightSafety International pilot training course, inform flight crews that, when the propeller electronic control (PEC) detects a failure condition that requires propeller feathering, the ability to subsequently unfeather the propeller will be lost, even in the case of a greater emergency. Information was not recovered from the cockpit voice recorder (CVR) since it was overwritten while external electrical power was applied to the aircraft following the incident flight.Other Finding Information was not recovered from the cockpit voice recorder (CVR) since it was overwritten while external electrical power was applied to the aircraft following the incident flight. Following this incident, Transport Canada (TC) cancelled the flight permit until it could be shown that adequate inspections had determined that the aircraft was safe to operate. A new flight permit was issued on 10June2005 for the balance of the flight test program. Cascade Aerospace Inc. instituted a self-imposed restriction limiting the balance of the flight test program to an intentional manoeuvring minimum limit of +0.5g as opposed to the previous self-imposed limit of 0g. On 21 July 2005, the TSB Pacific regional office distributed two occurrence bulletins. One addressed the issue of the availability of information regarding the engine pitch-up limitation. The other addressed the issue of the availability of information to flight crews regarding the inability to unfeather a propeller subsequent to a fault condition that requires a propeller to be feathered.Safety Action Taken Following this incident, Transport Canada (TC) cancelled the flight permit until it could be shown that adequate inspections had determined that the aircraft was safe to operate. A new flight permit was issued on 10June2005 for the balance of the flight test program. Cascade Aerospace Inc. instituted a self-imposed restriction limiting the balance of the flight test program to an intentional manoeuvring minimum limit of +0.5g as opposed to the previous self-imposed limit of 0g. On 21 July 2005, the TSB Pacific regional office distributed two occurrence bulletins. One addressed the issue of the availability of information regarding the engine pitch-up limitation. The other addressed the issue of the availability of information to flight crews regarding the inability to unfeather a propeller subsequent to a fault condition that requires a propeller to be feathered.